Light-emitting device with internal non-specular light redirection and anti-reflective exit surface

ABSTRACT

A light-emitting device includes a semiconductor diode structure with one or more light-emitting active layers, an anti-reflection coating on its front surface, and a redirection layer on its back surface. Active-layer output light propagates within the diode structure. The anti-reflection coating on the front surface increases transmission of active-layer output light incident below the critical angle Θ C . Active-layer output light incident on the redirection layer at an incidence angle greater than Θ C  is redirected to propagate toward the front surface at an incidence angle that is less than Θ C . Device output light is transmitted by the front surface to propagate in an ambient medium, and includes first and second portions of the active-layer output light incident on the front surface at an incidence angle less than Θ C , the first portion without redirection by the redirection layer and the second portion with redirection by the redirection layer.

BENEFIT CLAIMS TO RELATED APPLICATIONS

This application claims benefit of U.S. provisional App. No. 62/948,704entitled “Light-emitting device with internal non-specular lightredirection and anti-reflective exit surface” filed Dec. 16, 2019 in thenames of Antonio Lopez-Julia, Venkata Ananth Tamma, Aimi Abass, andPhilipp Schneider. This application claims priority of EP App. No.20160667.0 entitled “Light-emitting device with internal non-specularlight redirection and anti-reflective exit surface” filed Mar. 3, 2020in the names of Antonio Lopez-Julia, Venkata Ananth Tamma, Aimi Abass,and Philipp Schneider. Both of said applications are incorporated byreference as if set forth herein in their entireties.

FIELD OF THE INVENTION

The field of the present invention relates to light-emitting devices.Devices are disclosed herein that include internal, non-specular lightredirection and an anti-reflective exit surface, resulting in enhancedlight extraction efficiency.

BACKGROUND

Typical light-emitting diodes (LEDs) include an internal active layerwithin a semiconductor diode structure that emits light when driven byan electric current. The back surface of the semiconductor diodestructure (and in some instances one or more or all side surfaces)typically includes a reflector that reflects light incident within thesemiconductor diode structure to propagate toward the front surface ofthe diode structure (the front surface can also be referred to herein asthe exit surface). Many semiconductor materials have relatively largerefractive indices (often around 3 or more) which would result in alarge fraction of the emitted light being trapped within thesemiconductor diode structure by total internal reflection. In someconventional light-emitting diodes texturing (e.g., corrugations, bumpsor dimples, or other surface features or roughness) is formed on orattached to the front surface of the semiconductor diode structure (alsoreferred to as the exit surface). The back-surface reflector in suchso-called cavity emitters ensures that nearly all light propagatingwithin the semiconductor diode structure eventually is incident on thefront surface. The front-surface texturing serves to spoil totalinternal reflection at least partly, allowing a portion of the emittedlight to escape the semiconductor diode structure through the frontsurface as device output light, while redirecting other portionspropagate to within the semiconductor diode structure in directions thatdiffer from that of a specular reflection from a flat exit surface.Those redirected portions eventually reach the front surface again andhave another opportunity to escape by transmission through the frontsurface. This light recirculation process continues, and each so-called“photon bounce” (i.e., each round trip back and forth between the frontand back surfaces through the semiconductor diode structure) within theeffective “LED cavity” formed by the back-surface reflector and thefront-surface texturing increases the overall probability of that photonescaping through the front surface as device output light.

One example of a conventional LED cavity emitter 10 is illustratedschematically in FIG. 6 and includes a semiconductor diode structure 12,a light-emitting active layer 14 within the semiconductor diodestructure 12, texturing 16 on the front surface of the semiconductordiode structure 12, and a reflector 18 on the back surface of thesemiconductor diode structure 12. Another example of a conventional LEDcavity emitter 20 is illustrated schematically in FIG. 7 and includes asemiconductor diode structure 12, a light-emitting active layer 14within the semiconductor diode structure 12, texturing 16 on the frontsurface of the diode structure 12, a dielectric layer 19 on the backsurface of the diode structure 12, and a reflector 18 on the backsurface of the dielectric layer 19.

The practical reality, however, is that the probability per bounce of aphoton being transmitted through the front surface is relatively low,which in turn requires a relatively large number of round trips toachieve a sufficiently high probability of photon extraction (e.g., 10to 50 bounces to achieve extraction efficiency approaching 90%,depending on the particular materials employed). That relatively highnumber of round trips or photon bounces in turn requires sufficientlylow optical loss per round trip through the semiconductor diodestructure (e.g., loss due to absorption by diode structure, activelayer, or reflector materials, or insufficient reflectivity of thereflector). That low-loss requirement in some cases can drive up thecost or complexity of the light-emitting device (e.g.: use of a silverreflector instead of aluminum; use of a multi-layer thin film reflectorinstead of a metal reflector; or use of higher-purity materials fordiode structure, active layer, or reflector), or result in devices withlow extraction efficiency (e.g., if low-loss alternatives areunavailable or cost-prohibitive).

SUMMARY

An inventive light-emitting device comprises a semiconductor diodestructure having front and back surfaces, one or more light-emittingactive layers within the semiconductor diode structure, ananti-reflection coating on the front surface, and a redirection layer onthe back surface. The front surface is positioned against an ambientmedium, and that interface is characterized by a critical angle Θc at anominal vacuum wavelength λ₀. The one or more active layers emitactive-layer output light characterized by the nominal vacuum wavelengthλ₀ to propagate within the semiconductor diode structure. Theanti-reflection coating on the front surface of the semiconductor diodestructure exhibits reflectivity, for light incident on the front surfacewithin the semiconductor diode structure at an incidence angle less thanΘ_(C) and at the nominal vacuum wavelength λ₀, that is less thancorresponding Fresnel reflectivity between semiconductor diode structurematerial and the ambient medium without the anti-reflection coating. Theredirection layer includes one or more of (i) an array of nano-antennae,(ii) a partial photonic bandgap structure, (iii) a photonic crystal,(iv) an array of meta-atoms or meta-molecules, or (v) a diffusebackscatterer. Some active-layer output light, incident within thesemiconductor diode structure on the redirection layer at an incidenceangle greater than Θ_(C), is redirected by the redirection layer topropagate toward the front surface of the semiconductor diode structureat an incidence angle with respect to the front surface that is lessthan Θ_(C). Device output light is transmitted by the front surface topropagate in the ambient medium. The device output light includes firstand second portions of the active-layer output light propagating withinthe semiconductor diode structure and incident on the front surfacewithin the diode structure at an incidence angle less than Θ_(C), thefirst portion without redirection by the redirection layer and thesecond portion with redirection by the redirection layer.

In some examples the inventive light-emitting device can exhibit aphoton extraction efficiency that is greater than about 80.%, greaterthan about 90.%, or greater than about 95.%. In some examples theinventive light-emitting device can exhibit a mean number ofredirections per photon emitted by the active layer, by the redirectionsurface before transmission by the front surface, that is less than 30,less than 20, less than 10, or less than 5.

In some examples the redirection layer can exhibit an efficiency ofredirection, of the light incident within the semiconductor diodestructure on the redirection layer at an incidence angle greater thanΘ_(C) to propagate toward the front surface of the diode structure at anincidence angle with respect to the front surface that is less thanΘ_(C), that is greater than about 80.%, greater than about 85.%, greaterthan about 90.%, or greater than about 95.%, so that all lightrecirculation takes place between the front and back surfaces of thesemiconductor diode structure. In some other examples, thelight-emitting device can further include a lower-index dielectric layeron the back surface of the redirection layer, and a reflective layer onthe back surface of the dielectric layer; in such examples at least somelight recirculation occurs between the front surface and the reflectivelayer and at least partly within the dielectric layer.

Objects and advantages pertaining to light-emitting devices may becomeapparent upon referring to the example embodiments illustrated in thedrawings and disclosed in the following written description or appendedclaims.

This Summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used as an aid in determining the scope of the claimed subjectmatter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of an example of an inventivelight-emitting device.

FIG. 2 illustrates schematically an example of redirection ofactive-layer output light.

FIG. 3 is a schematic cross-sectional view of another example of aninventive light-emitting device.

FIG. 4 illustrates schematically another example of redirection ofactive-layer output light.

FIGS. 5A through 5D illustrate schematically several examples ofnano-antennas.

FIG. 6 is a schematic cross-sectional view of an example of aconventional light-emitting device.

FIG. 7 is a schematic cross-sectional view of another example of aconventional light-emitting device.

The embodiments depicted are shown only schematically; all features maynot be shown in full detail or in proper proportion; for clarity certainfeatures or structures may be exaggerated or diminished relative toothers or omitted entirely; the drawings should not be regarded as beingto scale unless explicitly indicated as being to scale. In particular,the height, depth, or width of various elements often can be exaggeratedrelative to other elements or, e.g., the thickness of an underlyingsubstrate or diode structure. The embodiments shown are only examplesand should not be construed as limiting the scope of the presentdisclosure or appended claims.

DETAILED DESCRIPTION OF EMBODIMENTS

It would be desirable to provide an inventive light-emitting device thatexhibits a relatively high photon extraction efficiency (e.g., 80% ormore) and a relatively reduced number of photon bounces (e.g., 30 orfewer).

For purposes of the present disclosure and appended claims, “incidenceangle” and “angle of incidence” of light incident on a surface orinterface refers to the angle between the propagation direction of theincident light and a vector normal to the surface or interface.Accordingly, light propagating at normal incidence with respect to asurface would have an incidence angle of 0°, while light propagatingnear grazing incidence with respect to that surface would have anincidence angle approaching 90°. For purposes of the present disclosureand appended claims, the “critical angle” for light incident on asurface or interface between media of differing refractive indicesrefers to the incidence angle, for light propagating within the higherindex medium, above which the light undergoes total internal reflectionwithin the higher-index medium. For purposes of the present disclosureand appended claims, “oblique light” or “oblique radiation” shall referto light propagating within a substrate or diode structure at incidenceangles greater than Θ_(C) with respect to the front and back surfacesthereof, while “perpendicular light” or “perpendicular radiation” shallrefer to light propagating within a substrate or diode structure atincidence angles less than Θ_(C) with respect to those surfaces, even ifnot literally perpendicular; “normal” shall be reserved for lightincident at an incidence angle of 90°. For purposes of the presentdisclosure and appended claims, any arrangement of a layer, surface,substrate, diode structure, or other structure “on,” “over,” or“against” another such structure shall encompass arrangements withdirect contact between the two structures as well as arrangementsincluding some intervening structure between them. Conversely, anyarrangement of a layer, surface, substrate, diode structure, or otherstructure “directly on,” “directly over,” or “directly against” anothersuch structure shall encompass only arrangements with direct contactbetween the two structures.

A first example of an inventive light-emitting device 100 is illustratedschematically in FIG. 1 and includes a semiconductor diode structure102, one or more light-emitting active layers 104, an anti-reflectioncoating 106 on the front surface (i.e., the exit surface) of thesemiconductor diode structure 102, and a redirection layer 108 on theback surface of the semiconductor diode structure 102. The front surfaceof the semiconductor diode structure 102 is positioned against anambient medium 99 and characterized by a critical angle Θ_(C) at anominal vacuum wavelength λ₀. In many typical examples the semiconductordiode structure includes reflective coatings on one or more or alllateral surfaces thereof; those lateral reflective coatings can be ofany suitable type or arrangement, and act to reflect any light incidentthereon within the semiconductor diode structure 102. One or morelight-emitting active layers 104 within the semiconductor diodestructure 102 are arranged so as to emit active-layer output light (atthe nominal vacuum wavelength λ₀) to propagate within the semiconductordiode structure 102. “At a nominal vacuum wavelength λ₀” means that thewavelength spectrum of the device output light includes a range ofwavelengths that includes λ₀. In many examples the device output lightis within about ±5 nm or about ±10 nm of λ₀; in other examples thespectral width of the device output light can be greater than that.

In some examples the front surface of the semiconductor diode structure102 is positioned against an ambient medium 99 that is vacuum, air, agaseous medium, or a liquid medium. In some examples the ambient medium99 includes one or more substantially solid materials among: doped orundoped silicone, or one or more doped or undoped polymers. In someexamples the nominal output vacuum wavelength λ₀ is larger than about0.20 μm, larger than about 0.4 μm, larger than about 0.8 μm, smallerthan about 10. μm, smaller than about 2.5 μm, or smaller than about 1.0μm. In some examples the light-emitting device 100 comprises alight-emitting diode. In some examples the semiconductor diode structure102 or active layer 104 includes one or more doped or undoped III-V,II-VI, or Group IV semiconductor materials or alloys or mixturesthereof. Note that the semiconductor diode structure 102 and activelayer(s) 104 typically are formed together by a process sequence, oftenwith the active layer(s) 104 formed on a surface of an initialsemiconductor layer or substrate and then additional semiconductormaterial deposited over the active layers; other suitable fabricationsequences can be employed. In some examples the light-emitting layer 104includes one or more doped or undoped III-V, II-VI, or Group IVsemiconductor materials or alloys or mixtures thereof. In some examplesthe light-emitting layer 104 includes one or more p-n junctions, one ormore quantum wells, one or more multi-quantum wells, or one or morequantum dots. The light emitting device 100 typically includes one ormore electrodes (not shown) for delivering electric current to theactive layer 104 to produce the active-layer output light.

For light at the nominal vacuum wavelength λ₀ that is incident on thefront surface within the semiconductor diode structure 102 at anincidence angle less than Θ_(C) (i.e., for perpendicular light), theanti-reflection coating 106 on the front surface of the semiconductordiode structure 102 exhibits reflectivity that is less thancorresponding Fresnel reflectivity between semiconductor material andthe ambient medium 99 without the anti-reflection coating 106. Theanti-reflection coating 106 can be of any suitable type or arrangement.In some examples the anti-reflection coating 106 includes a single-layerquarter-wave dielectric thin film. In some examples the anti-reflectioncoating 106 includes a multi-layer dielectric thin film. In someexamples the anti-reflection coating 106 includes a moth-eye structureor other similar nanostructured film, or a gradient-index film. Inprinciple the reflectivity of the front surface of the semiconductordiode structure 102 is most desirably reduced to zero. In practice, insome examples the anti-reflection coating 106 can reduce thereflectivity of the front surface of the semiconductor diode structure102 to less than about 10.%, less than about 5.%, less than about 2.0%.less than about 1.0%, or less than about 0.5%. Contrast that withFresnel reflectivity (at normal incidence) of an interface between asemiconductor and air (about 25%) or between a semiconductor andsilicone (about 11%).

The redirection layer 108 on the back surface of the semiconductor diodestructure 102 includes one or more of (i) an array of nano-antennae,(ii) a partial photonic bandgap structure, (iii) a photonic crystal,(iv) an array of meta-atoms or meta-molecules (described further below),or (v) a diffuse backscatterer. The redirection layer 108 is arranged sothat at least a portion of light incident within the semiconductor diodestructure 102 at an incidence angle greater than Θ_(C) (i.e., obliquelight) is redirected to propagate within the semiconductor diodestructure 102 at an angle less than Θ_(C) (i.e., as perpendicularlight). By the reciprocity of Maxwell's equations, such a redirectionlayer 108 will also redirect perpendicular light to propagate as obliquelight. An effect of the anti-reflection coating 106 on the front surfaceof the semiconductor diode structure 102 is to enable perpendicularlight propagating within the semiconductor diode structure 102 to escapeby transmission through the front surface as device output light insteadof being reflected by the front surface and redirected as oblique lightby the redirection layer 108. In principle that redirected oblique lightcan be redirected again to propagate as perpendicular light and escapethrough the front surface, but at the expense of another round tripthrough the semiconductor diode structure 102.

In the example of FIG. 1, a first fraction of the active-layer outputlight propagates as perpendicular light directly from the active layer104 to the front surface and is mostly transmitted (depending on theeffectiveness of the anti-reflection coating 106) as a first portion ofthe device output light. A second fraction of the active-layer outputlight propagates as oblique light within the semiconductor diodestructure 102 from the active layer 104 and is incident on theredirection layer 108, with or without one or more total internalreflections from front or lateral surfaces of the semiconductor diodestructure 102. As depicted schematically in FIG. 2, the redirectionlayer 108 redirects at least a portion of incident oblique light topropagate toward the front surface of the semiconductor diode structure102 as perpendicular light, which is mostly transmitted (depending onthe effectiveness of the anti-reflection coating 106) as a secondportion of the device output light. A third fraction of the active-layeroutput light propagates as perpendicular light within the semiconductordiode structure 102 directly from the active layer 104 to theredirection layer 108. Due to the reciprocity of Maxwell's equations,the redirection layer 108 redirects at least a portion of theperpendicular light to propagate toward the front surface of thesemiconductor diode structure 102 as oblique light. After a round tripthrough the semiconductor diode structure 102 and back, including totalinternal reflection at the front surface of the semiconductor diodestructure 102, the third fraction is incident again on the redirectionsurface 108 as oblique light, redirected to propagate toward the frontsurface as perpendicular light, and mostly escapes by transmissionthrough the front surface of the semiconductor diode structure 102.

In an idealized case wherein the front surface reflects no perpendicularlight and 100% of oblique light, the redirection layer redirects with100% efficiency, and there are no optical losses, every photon emittedby the active layer 104 will exit the front surface of the semiconductordiode structure 102 as device output light after at most two round tripsthrough the semiconductor diode structure 102 and two redirections bythe redirection surface 108. In reality, of course, none of thosecondition is fully met. Despite the anti-reflection coating 106, thefront surface will nevertheless reflect some small fraction ofperpendicular light, resulting in at least two additional round tripsfor those photons. The front surface of the semiconductor diodestructure 102 might absorb or scatter some small fraction of incidentlight contributing to round-trip optical loss. The lateral surfaces ofthe semiconductor diode structure 102 might absorb, scatter, or transmitsome small fraction of incident light, contributing to round-tripoptical loss. Such optical losses arising from the front and lateralsurfaces of the semiconductor diode structure 102 result in somefraction of emitted photons that never exit the semiconductor diodestructure 102 (at least not in the desired output direction) and reducedextraction efficiency. If sufficiently low, such optical losses can betolerated, if the resulting extraction efficiency is sufficiently high.

Perhaps most significantly, the redirection layer 108 typically willexhibit less than 100% redirection efficiency (defined as the fractionof incident oblique light redirected to propagate as perpendicularlight, or vice versa). The nature of the deviation from 100% redirectionefficiency is significant, as well as its magnitude. If the light thatis not suitably redirected still propagates within the semiconductordiode structure (e.g., perpendicular light redirected as a mixture ofoblique and perpendicular light (advantageous), or oblique lightredirected as a mixture of oblique and perpendicular light(disadvantageous)), then there will be further opportunities forconversion to forward-propagating perpendicular light and eventualescape from the semiconductor diode structure 102 as device outputlight. An average number of round trips required to reach a givenprobability for photon escape depends on the specific fractions ofoblique and perpendicular light produced by each interaction with theredirection surface 108. If round-trip optical losses are sufficientlylow, those additional round trips can be tolerated; such scenarios canyield inventive light-emitting devices with adequate extractionefficiencies. More problematic is a redirection layer 108 that is lossy(e.g., by absorption, scattering, or transmission). Any optical lossarising from the redirection layer 108 results in photons that neverexit the semiconductor diode structure 102 (at least not in the desiredoutput direction) and reduced extraction efficiency. Again, ifsufficiently low, such optical loss exhibited by the redirection layer108 can be tolerated, if the resulting extraction efficiency issufficiently high.

In some examples arranged as in FIG. 1 the redirection layer 108exhibits an efficiency of redirection, of the light incident within thesemiconductor diode structure on the redirection layer at an incidenceangle greater than Θ_(C) to propagate toward the front surface of thesemiconductor diode structure at an incidence angle with respect to thefront surface that is less than Θ_(C), that is greater than about 80.%,greater than about 85.%, greater than about 90.%, or greater than about95.%. In some examples arranged as in FIG. 1 the redirection layer 108exhibits optical loss per pass for light incident thereon that is lessthan about 20.%, less than about 10.%, less than about 5.%, less thanabout 2.0%, or less than about 1.0%. Design or optimization of theredirection layer 108 can be performed (by calculation, simulation, oriterative designing/making/testing of prototypes or test devices),typically with increased overall extraction efficiency as a primary orsecondary figure-of-merit. Instead or in addition, reduction of cost ormanufacturing complexity can be employed as primary or secondaryfigures-of-merit in such design or optimization processes.

A second example of an inventive light-emitting device 100 isillustrated schematically in FIG. 3 and includes a semiconductor diodestructure 102, one or more light-emitting active layers 104, ananti-reflection coating 106 on the front surface (i.e., the exitsurface) of the semiconductor diode structure 102, and a redirectionlayer 108 on the back surface of the semiconductor diode structure 102.In this example the device 100 further includes a dielectric layer 110and a reflective layer 112. The dielectric layer 110 is positioned onthe back surface of the redirection layer 108 (with the redirectionlayer 108 between the dielectric layer 110 and the back surface of thesemiconductor diode structure 102), and is substantially transparent andcharacterized by a refractive index lower than that of the semiconductordiode structure (at the nominal vacuum wavelength λ₀). The reflectivelayer 112 is positioned on the back surface of the dielectric layer 110(with the dielectric layer 110 between the reflective layer 112 and theback surface of the redirection layer 108). The semiconductor diodestructure 102, active layer 104, and anti-reflection coating 106 can bearranged in any suitable way with any suitable material composition,including those described above. The reflective layer 112 can exhibitreflectivity, at the nominal vacuum wavelength λ₀, that is greater thanabout 90.%, greater than about 95.%, or greater than about 98.%.Typically, larger reflectivity will result in higher extractionefficiency. The reflective layer 112 can be of any suitable type orarrangement, and can include one or more materials among: doped orundoped silicon; one or more doped or undoped III-V, II-VI, or Group IVsemiconductors; doped or undoped silicon oxide, nitride, or oxynitride;one or more doped or undoped metal oxides, nitrides, or oxynitrides; oneor more optical glasses; one or more doped or undoped polymers; or oneor more metals or metal alloys. In some examples, the reflective layer112 includes a metallic coating or a dielectric coating (e.g., amulti-layer dielectric thin film). The dielectric layer 110 can be a fewhundred nanometers thick up to several micrometers thick, and caninclude one or more materials among: doped or undoped silicon; one ormore doped or undoped III-V, II-VI, or Group IV semiconductors; doped orundoped silicon oxide, nitride, or oxynitride; one or more doped orundoped metal oxides, nitrides, or oxynitrides; one or more opticalglasses; or one or more doped or undoped polymers.

In the example of FIG. 3, oblique light emitted by the active layer 104propagates within the semiconductor diode structure 102, and isreflected or redirected within the semiconductor diode structure 102 (bythe front surface, lateral surfaces, or the redirection layer 108) asdescribed above. However, instead of representing optical loss (as inthe example of FIG. 1), light that is transmitted by the redirectionlayer 108 propagates within the dielectric layer 110, is reflected bythe reflective layer 112, and propagates forward toward the redirectionlayer 108. That particular optical loss mechanism of the arrangement ofFIG. 1 therefore can be mitigated by the presence of the dielectriclayer 110 and the reflective layer 112. The redirection layer 108 can bearranged (e.g., as in FIG. 4) so as to exhibit (i) preferentialredirection of oblique light incident within the semiconductor diodestructure 102 to propagate as perpendicular light within thesemiconductor diode structure 102, and (ii) preferential redirection ofoblique light incident within the dielectric layer 110 to propagate asperpendicular light within the semiconductor diode structure 102. Theredirection layer 108 can be arranged so as to exhibit redirectionefficiency for those processes that is greater than about 20.%, greaterthan about 40.%, greater than about 60.%, or greater than about 80.%. Insome examples it may be possible to maximize or optimize efficiencies ofboth of those processes simultaneously. More typically such simultaneousoptimization is not possible, and a compromise structure for theredirection layer can be chosen that exhibits sufficient redirectionefficiency for each process to provide a necessary, desirable, orsuitable extraction efficiency. Redirection of any perpendicular lightpropagating in the semiconductor diode structure 102 or in thedielectric layer 110 can also be considered when designing or optimizingthe redirection layer 108.

In some examples arranged as in FIG. 3, the back surface of thedielectric layer 110 includes corrugations, dimples, bumps, protrusions,or depressions; those surface features can arise during growth ordeposition of the dielectric layer 110 on the redirection layer 108, ifthe morphology of the redirection layer 108 is not planar. That is oftenthe case, e.g., when the redirection layer includes an array ofnano-antennae, meta-atoms, or meta-molecules. That topology is reflectedto some degree (typically somewhat smoothed out) in the surface of thedielectric layer. Deposition or growth of the reflective layer 112 oftenresults in roughly conformal coverage of the surface of the dielectriclayer 110. The non-planarity of the dielectric layer 110 can beadvantageous, in that reflection from that surface will tend to mixoblique and perpendicular light propagating within the dielectric layer,which in some instances can enhance overall extraction efficiency. Insome examples, such non-planarity of the dielectric layer 110 can beimparted by design, e.g., to a degree greater than that arising from thepresence of the redirection layer 108 during deposition or growth of thedielectric layer 110.

Whichever of the arrangements described above is employed, in someexamples the light-emitting device 100 can exhibit an extractionefficiency that is greater than about 80.%, greater than about 90.%, orgreater than about 95.%. In some examples the light-emitting device 100exhibits a mean number of redirections per photon emitted by the activelayer 104 (by the redirection surface 108 or, if present, the reflectivelayer 112), before transmission by the front surface, that is less than30, less than 20, less than 10, or less than 5.

In some examples the redirection layer 108 can include an array ofnano-antennae 109. Several examples are illustrated schematically inFIGS. 5A-5D, in which the nano-antennae 109 are shown extending from thesemiconductor diode structure 102 into a medium 98 (e.g., an encapsulantor ambient medium against the redirection layer 108); in other examples(not shown) the nano-antennae 109 can extend from the surface of thesemiconductor diode structure 102 into the dielectric layer 110, or canextend from the surface into the semiconductor material of the diodestructure 102. The nano-antennae can include one or more antennamaterials, and can be shaped, sized and spaced relative to the nominaloutput vacuum wavelength λ₀, and arranged along the redirection layer108) so as to reradiate, upon irradiation by active-layer output lightat an incidence angle greater than the critical angle Θ_(C), at least aportion of the active-layer output light so as to result collectively inredirection of that active-layer output light to propagate at anincidence angle less than Θ_(C). Any suitable sizes, spacing, materials(e.g., silicon or TiO₂), antenna shapes (e.g., cylindrical,frusto-conical, frusto-pyramidal, horizontal dimers, vertical dimers,coaxial dimers, and so forth; as in, e.g., FIGS. 5A-5D), andarrangements (e.g., triangular grid, rectangular grid, hexagonal grid,other grids, or an irregular, aperiodic, or random arrangement) can beemployed. Typically, calculation or computer simulation is required toachieve at least a preliminary design for a nano-antennae array; a finaldesign can typically be achieved by iterative experimental optimizationof the various parameters by fabricating and characterizinglight-emitting device incorporating test arrays in their correspondingredirection layers 108. Note that an array that is not necessarily fullyoptimized can nevertheless provide redirection efficiency adequate toprovide an acceptably high extraction efficiency for the light-emittingdevice 100; such partly optimized arrays fall within the scope of thepresent disclosure or appended claims. Examples of suitablenano-antennae arrays can be found in, e.g., (i) Li et al,“All-Dielectric Antenna Wavelength Router with Bidirectional Scatteringof Visible Light,” Nano Letters, 16 4396 (2016), and (ii) (i) Shibanumaet al, “Experimental Demonstration of Tunable Directional Scattering ofVisible Light from All-Dielectric Asymmetric Dimers,” ACS Photonics, 4489 (2017), which are incorporated by reference as if fully set forthherein.

In some examples, the redirection layer 108 can include a partialphotonic bandgap structure arranged with one or more materials,morphology, and spacing relative the nominal output vacuum wavelengthλ₀, so as to redirect, upon irradiation by active-layer output light atan incidence angle greater than the critical angle Θ_(C), at least aportion of the active-layer output light to propagate at an incidenceangle less than Θ_(C). In some examples the redirection layer 108 caninclude a photonic crystal arranged with one or more materials, crystalmorphology, and crystal-lattice spacing relative the nominal outputvacuum wavelength λ₀, so as to redirect, upon irradiation byactive-layer output light at an incidence angle greater than thecritical angle Θ_(C), at least a portion of the active-layer outputlight to propagate at an incidence angle less than Θ_(C). In someexamples the redirection layer can include an array of meta-atoms ormeta-molecules that are composed of one or more meta-materials, sizedrelative to the nominal output vacuum wavelength λ₀, arranged along theredirection layer 108 and spaced relative to the nominal output vacuumwavelength λ₀, and shaped so as to reradiate, upon irradiation byactive-layer output light at an incidence angle greater than thecritical angle Θ_(C), at least a portion of the active-layer outputlight so as to result collectively in redirection of that active-layeroutput light to propagate at an incidence angle less than Θ_(C). In someexamples the redirection layer can include an efficient diffusebackscatterer or any suitable type or arrangement so as to backscatter,upon irradiation by active-layer output light at an incidence anglegreater than the critical angle Θ_(C), at least a portion of theincident active-layer output light to propagate at an incidence angleless than Θ_(C). In any of those examples, calculation or simulationfollowed by iterative experimental optimization (or at least partialoptimization) can be employed, in a manner similar to that describedabove.

In any of the arrangements described above for the redirection layer108, the redirection layer 108 can include one or more materials among:doped or undoped silicon; one or more doped or undoped III-V, II-VI, orGroup IV semiconductors; doped or undoped silicon oxide, nitride, oroxynitride; one or more doped or undoped metal oxides, nitrides, oroxynitrides; one or more optical glasses; one or more doped or undopedpolymers; or one or more metals or metal alloys.

Methods for making an inventive light-emitting device 100 include: (a)forming within the semiconductor diode structure 102 the one or morelight-emitting layers 104; (b) forming on the front surface of thesemiconductor diode structure 102 the anti-reflection coating 106; (c)forming on the back surface of the semiconductor diode structure 102 theredirection layer 108. A method can further include: (d) forming on theback surface of the redirection layer 108 the dielectric layer 110; and(e) forming on the back surface of the dielectric layer 110 thereflective layer 112. Any suitable one or more fabrication or materialprocessing techniques can be employed, in particular for forming theactive layers 104 and the redirection layer 108 in any suitablearrangement (including all of those described above). Suitabletechniques can include, but are not limited to, layer growth, masked ornon-masked deposition, masked or non-masked lithography, masked ornon-masked wet or dry etching, epitaxy, direct-write, self-assembly, andso forth. Which one or more techniques are suitable, desirable, ornecessary depends on the nature of the active layer 104 and theredirection surface 108 (e.g., p-n junction, multi-quantum well,nano-antenna array, meta-molecules, or other).

A method for operating an inventive light-emitting device 100 includessupplying electrical power to the light-emitting device 100 so that itemits device output light from the front surface of the semiconductordiode structure 102 to propagate in the ambient medium 99.

In addition to the preceding, the following example embodiments fallwithin the scope of the present disclosure or appended claims:

Example 1. A semiconductor light-emitting device comprising: (a) asemiconductor diode structure having front and back surfaces, the frontsurface being characterized, with respect to an ambient medium, by acritical angle Θ_(C) at a nominal vacuum wavelength λ₀; (b) one or morelight-emitting active layers within the semiconductor diode structurethat are arranged so as to emit active-layer output light at the nominalvacuum wavelength λ₀ to propagate within the semiconductor diodestructure; (c) an anti-reflection coating on the front surface of thesemiconductor diode structure that is arranged so that the front surfaceexhibits reflectivity, for light incident on the front surface withinthe semiconductor diode structure at an incidence angle less than Θ_(C),at the nominal vacuum wavelength λ₀, and with the front surface againstthe ambient medium, that is less than corresponding Fresnel reflectivityof an interface between semiconductor diode structure material and theambient medium without the anti-reflection coating; and (d) aredirection layer on the back surface of the semiconductor diodestructure, the redirection layer including one or more of (i) an arrayof nano-antennae, (ii) a partial photonic bandgap structure, (iii) aphotonic crystal, or (iv) an array of meta-atoms or meta-molecules, theredirection layer being structurally arranged so as to redirect at leasta portion of active-layer output light, incident within thesemiconductor diode structure on the redirection layer at an incidenceangle greater than Θ_(C), to propagate toward the front surface of thesemiconductor diode structure at an incidence angle with respect to thefront surface that is less than Θ_(C), (e) the semiconductorlight-emitting device exhibiting transmission by the front surface, asdevice output light that propagates in the ambient medium, of first andsecond portions of the active-layer output light propagating within thesemiconductor diode structure, the first portion without redirection bythe redirection layer and the second portion with redirection by theredirection layer.

Example 2. The device of Example 1 wherein the light-emitting deviceexhibits a photon extraction efficiency that is greater than about 80.%,greater than about 90.%, or greater than about 95.%.

Example 3. The device of any one of Examples 1 or 2 wherein thelight-emitting device exhibits a mean number of redirections per photonemitted by the active layer, by the redirection surface beforetransmission by the front surface, that is less than 30, less than 20,less than 10, or less than 5.

Example 4. The device of any one of Examples 1 through 3 wherein theredirection layer exhibits a redirection efficiency, of the lightincident within the semiconductor diode structure on the redirectionlayer at an incidence angle greater than Θ_(C) to propagate toward thefront surface of the semiconductor diode structure at an incidence anglewith respect to the front surface that is less than Θ_(C), that isgreater than about 80.%, greater than about 85.%, greater than about90.%, or greater than about 95.%.

Example 5. The device of Example 4 wherein the redirection layerexhibits optical loss per pass for light incident thereon that is lessthan about 20.%, less than about 10.%, less than about 5.%, less thanabout 2.0%, or less than about 1.0%.

Example 6. The device of any one of Examples 1 through 3 furthercomprising: (e) a dielectric layer on a back surface of the redirectionlayer with the redirection layer between the dielectric layer and theback surface of the semiconductor diode structure, the dielectric layerbeing, at the nominal vacuum wavelength λ₀, substantially transparentand characterized by a refractive index lower than that of thesemiconductor diode structure; and (f) a reflective layer on a backsurface of the dielectric layer with the dielectric layer between thereflective layer and the back surface of the redirection layer.

Example 7. The device of Example 6 wherein the redirection layerexhibits an efficiency of redirection, of the light incident within thesemiconductor diode structure on the redirection layer at an incidenceangle greater than Θ_(C) to propagate toward the front surface of thesemiconductor diode structure at an incidence angle with respect to thefront surface that is less than Θ_(C), that is greater than about 20.%,greater than about 40.%, greater than about 60.%, or greater than about80.%.

Example 8. The device of any one of Examples 6 or 7 wherein theredirection layer exhibits optical loss per pass for light incidentthereon that is less than about 10.%, less than about 5.%, less thanabout 2.0%, less than about 1.0%, or less than about 0.5%.

Example 9. The device of any one of Examples 6 through 8 wherein thereflective layer exhibits reflectivity, at the nominal vacuum wavelengthλ₀, that is greater than about 90.%, greater than about 95.%, or greaterthan about 98.%.

Example 10. The device of any one of Examples 6 through 9 wherein thereflective layer includes a metallic coating or a dielectric coating.

Example 11. The device of any one of Examples 6 through 10 wherein theback surface of the dielectric layer includes corrugations, dimples,bumps, protrusions, or depressions.

Example 12. The device of any one of Examples 6 through 11 wherein thedielectric layer includes one or more materials among: doped or undopedsilicon; one or more doped or undoped III-V, II-VI, or Group IVsemiconductors; doped or undoped silicon oxide, nitride, or oxynitride;one or more doped or undoped metal oxides, nitrides, or oxynitrides; oneor more optical glasses; or one or more doped or undoped polymers.

Example 13. The device of any one of Examples 6 through 12 wherein thereflective layer includes one or more materials among: doped or undopedsilicon; one or more doped or undoped III-V, II-VI, or Group IVsemiconductors; doped or undoped silicon oxide, nitride, or oxynitride;one or more doped or undoped metal oxides, nitrides, or oxynitrides; oneor more optical glasses; one or more doped or undoped polymers; or oneor more metals or metal alloys.

Example 14. The device of any one of Examples 1 through 13 wherein theredirection layer includes an array of nano-antennae that include one ormore antenna materials, are shaped, sized, and spaced relative to thenominal output vacuum wavelength λ₀, and arranged along the redirectionlayer, so as to reradiate, upon irradiation by active-layer output lightat an incidence angle greater than Θ_(C), at least a portion of theactive-layer output light to result collectively in the redirectionthereof to propagate toward the front surface of the semiconductor diodestructure at an incidence angle less than Θ_(C).

Example 15. The device of any one of Examples 1 through 14 wherein theredirection layer includes a partial photonic bandgap structure arrangedwith one or more materials, morphology, and spacing relative the nominaloutput vacuum wavelength λ₀, so as to redirect, upon irradiation byactive-layer output light at an incidence angle greater than Θ_(C), atleast a portion of the active-layer output light to propagate toward thefront surface of the semiconductor diode structure at an incidence angleless than Θ_(C).

Example 16. The device of any one of Examples 1 through 15 wherein theredirection layer includes a photonic crystal arranged with one or morematerials, crystal morphology, and crystal-lattice spacing relative thenominal output vacuum wavelength λ₀, so as to redirect, upon irradiationby active-layer output light at an incidence angle greater than Θ_(C),at least a portion of the active-layer output light to propagate towardthe front surface of the semiconductor diode structure at an incidenceangle less than Θ_(C).

Example 17. The device of any one of Examples 1 through 16 wherein theredirection layer includes an array of meta-atoms or meta-molecules thatinclude one or more meta-materials, are shaped, sized, and spacedrelative to the nominal output vacuum wavelength λ₀, and arranged alongthe redirection layer, so as to reradiate, upon irradiation byactive-layer output light at an incidence angle greater than Θ_(C), atleast a portion of the active-layer output light to result collectivelyin the redirection thereof to propagate toward the front surface of thesemiconductor diode structure at an incidence angle less than Θ_(C).

Example 18. The device of any one of Examples 1 through 17 wherein theredirection layer includes a diffuse backscatterer arranged so as tobackscatter, upon irradiation by active-layer output light at anincidence angle greater than the critical angle Θ_(C), at least aportion of the incident active-layer output light to propagate at anincidence angle less than Θ_(C).

Example 19. The device of any one of Examples 1 through 18 wherein theredirection layer includes one or more materials among: doped or undopedsilicon; one or more doped or undoped III-V, II-VI, or Group IVsemiconductors; doped or undoped silicon oxide, nitride, or oxynitride;one or more doped or undoped metal oxides, nitrides, or oxynitrides; oneor more optical glasses; one or more doped or undoped polymers; or oneor more metals or metal alloys.

Example 20. The device of any one of Examples 1 through 19 wherein thefront surface of the semiconductor diode structure is positioned againstthe ambient medium, and the ambient medium is substantially solid andincludes one or more materials among: doped or undoped silicone, or oneor more doped or undoped polymers.

Example 21. The device of any one of Examples 1 through 19 wherein thefront surface of the semiconductor diode structure is positioned againstthe ambient medium, and the ambient medium comprises vacuum, air, agaseous medium, or a liquid medium.

Example 22. The device of any one of Examples 1 through 21 wherein theanti-reflection coating includes a single-layer quarter-wave dielectricthin film or a multi-layer dielectric thin film.

Example 23. The device of any one of Examples 1 through 22 wherein theanti-reflection coating includes a moth-eye structure or anindex-gradient film.

Example 24. The device of any one of Examples 1 through 23 whereinreflectivity of the front surface of the semiconductor diode structurewith the anti-reflection coating is less than about 10.%, less thanabout 5.%, less than about 2.0%, less than about 1.0%, or less thanabout 0.5%.

Example 25. The device of any one of Examples 1 through 24 wherein theanti-reflection coating includes one or more materials among: doped orundoped silicon; one or more doped or undoped III-V, II-VI, or Group IVsemiconductors; doped or undoped silicon oxide, nitride, or oxynitride;one or more doped or undoped metal oxides, nitrides, or oxynitrides; oneor more optical glasses; or one or more doped or undoped polymers.

Example 26. The device of any one of Examples 1 through 25 wherein thenominal output vacuum wavelength λ₀ is larger than about 0.20 μm, largerthan about 0.4 μm, larger than about 0.8 μm, smaller than about 10. μm,smaller than about 2.5 μm, or smaller than about 1.0 μm.

Example 27. The device of any one of Examples 1 through 26 wherein thesemiconductor light-emitting device comprises a semiconductorlight-emitting diode.

Example 28. The device of any one of Examples 1 through 27 wherein thesemiconductor diode structure includes one or more doped or undopedIII-V, II-VI, or Group IV semiconductor materials or alloys or mixturesthereof.

Example 29. The device of any one of Examples 1 through 28 wherein thelight-emitting layer includes one or more doped or undoped III-V, II-VI,or Group IV semiconductor materials or alloys or mixtures thereof.

Example 30. The device of any one of Examples 1 through 29 wherein thelight-emitting layer includes one or more p-n junctions, one or morequantum wells, one or more multi-quantum wells, or one or more quantumdots.

Example 31. A method for making the light-emitting device of any one ofExamples 1 through 30, the method comprising: (a) forming within thesemiconductor diode structure the one or more light-emitting activelayers; (b) forming on the front surface of the semiconductor diodestructure the anti-reflection coating; and (c) forming on the backsurface of the semiconductor diode structure the redirection layer.

Example 32. The method of Example 31 further comprising: (d) forming onthe back surface of the redirection layer the dielectric layer with theredirection layer between the dielectric layer and the back surface ofthe semiconductor diode structure; and (e) forming on the back surfaceof the dielectric layer the reflective layer with the dielectric layerbetween the reflective layer and the back surface of the redirectionlayer.

Example 33. A method for operating the light-emitting device of any oneof Examples 1 through 30, the method comprising supplying to thelight-emitting device electrical power so that the light-emitting deviceemits device output light from the front surface of the semiconductordiode structure to propagate in the ambient medium against the frontsurface.

It is intended that equivalents of the disclosed example embodiments andmethods shall fall within the scope of the present disclosure orappended claims. It is intended that the disclosed example embodimentsand methods, and equivalents thereof, may be modified while remainingwithin the scope of the present disclosure or appended claims.

In the foregoing Detailed Description, various features may be groupedtogether in several example embodiments for the purpose of streamliningthe disclosure. This method of disclosure is not to be interpreted asreflecting an intention that any claimed embodiment requires morefeatures than are expressly recited in the corresponding claim. Rather,as the appended claims reflect, inventive subject matter may lie in lessthan all features of a single disclosed example embodiment. Thereforethe present disclosure shall be construed as implicitly disclosing anyembodiment having any suitable subset of one or more features—whichfeatures are shown, described, or claimed in the presentapplication—including those subsets that may not be explicitly disclosedherein. A “suitable” subset of features includes only features that areneither incompatible nor mutually exclusive with respect to any otherfeature of that subset. Accordingly, the appended claims are herebyincorporated into the Detailed Description, with each claim standing onits own as a separate disclosed embodiment. In addition, each of theappended dependent claims shall be interpreted, only for purposes ofdisclosure by said incorporation of the claims into the DetailedDescription, as if written in multiple dependent form and dependent uponall preceding claims with which it is not inconsistent. It should befurther noted that the cumulative scope of the appended claims can, butdoes not necessarily, encompass the whole of the subject matterdisclosed in the present application.

The following interpretations shall apply for purposes of the presentdisclosure and appended claims. The article “a” shall be interpreted as“one or more” unless “only one,” “a single,” or other similar limitationis stated explicitly or is implicit in the particular context;similarly, the article “the” shall be interpreted as “one or more ofthe” unless “only one of the,” “a single one of the,” or other similarlimitation is stated explicitly or is implicit in the particularcontext. The conjunction “or” is to be construed inclusively (e.g., “adog or a cat” would be interpreted as “a dog, or a cat, or both”; e.g.,“a dog, a cat, or a mouse” would be interpreted as “a dog, or a cat, ora mouse, or any two, or all three”), unless: (i) it is explicitly statedotherwise, e.g., by use of “either . . . or,” “only one of,” or similarlanguage; or (ii) two or more of the listed alternatives are mutuallyexclusive within the particular context, in which case “or” wouldencompass only those combinations involving non-mutually-exclusivealternatives. Similarly, “one or more of a dog or a cat” would beinterpreted as including (i) one or more dogs without any cats, (ii) oneor more cats without any dogs, or (iii) one or more dogs and one or morecats, unless explicitly stated otherwise or the alternatives areunderstood or disclosed (implicitly or explicitly) to be mutuallyexclusive or incompatible. Similarly, “one or more of a dog, a cat, or amouse” would be interpreted as (i) one or more dogs without any cats ormice, (ii) one or more cats without and dogs or mice, (iii) one or moremice without any dogs or cats, (iv) one or more dogs and one or morecats without any mice, (v) one or more dogs and one or more mice withoutany cats, (vi) one or more cats and one or more mice without any dogs,or (vii) one or more dogs, one or more cats, and one or more mice. “Twoor more of a dog, a cat, or a mouse” would be interpreted as (i) one ormore dogs and one or more cats without any mice, (ii) one or more dogsand one or more mice without any cats, (iii) one or more cats and one ormore mice without and dogs, or (iv) one or more dogs, one or more cats,and one or more mice; “three or more,” “four or more,” and so on wouldbe analogously interpreted. For any of the preceding recitations, if anypairs or combinations of the included alternatives are understood ordisclosed (implicitly or explicitly) to be incompatible or mutuallyexclusive, such pairs or combinations are understood to be excluded fromthe corresponding recitation. For purposes of the present disclosure andappended claims, the words “comprising,” “including,” “having,” andvariants thereof, wherever they appear, shall be construed as open endedterminology, with the same meaning as if a phrase such as “at least”were appended after each instance thereof, unless explicitly statedotherwise.

For purposes of the present disclosure or appended claims, when termsare employed such as “about equal to,” “substantially equal to,”“greater than about,” “less than about,” and so forth, in relation to anumerical quantity, standard conventions pertaining to measurementprecision and significant digits shall apply, unless a differinginterpretation is explicitly set forth. For null quantities described byphrases such as “substantially prevented,” “substantially absent,”“substantially eliminated,” “about equal to zero,” “negligible,” and soforth, each such phrase shall denote the case wherein the quantity inquestion has been reduced or diminished to such an extent that, forpractical purposes in the context of the intended operation or use ofthe disclosed or claimed apparatus or method, the overall behavior orperformance of the apparatus or method does not differ from that whichwould have occurred had the null quantity in fact been completelyremoved, exactly equal to zero, or otherwise exactly nulled.

For purposes of the present disclosure and appended claims, anylabelling of elements, steps, limitations, or other portions of anembodiment, example, or claim (e.g., first, second, third, etc., (a),(b), (c), etc., or (i), (ii), (iii), etc.) is only for purposes ofclarity, and shall not be construed as implying any sort of ordering orprecedence of the portions so labelled. If any such ordering orprecedence is intended, it will be explicitly recited in the embodiment,example, or claim or, in some instances, it will be implicit or inherentbased on the specific content of the embodiment, example, or claim. Inthe appended claims, if the provisions of 35 USC § 112(f) are desired tobe invoked in an apparatus claim, then the word “means” will appear inthat apparatus claim. If those provisions are desired to be invoked in amethod claim, the words “a step for” will appear in that method claim.Conversely, if the words “means” or “a step for” do not appear in aclaim, then the provisions of 35 USC § 112(f) are not intended to beinvoked for that claim.

If any one or more disclosures are incorporated herein by reference andsuch incorporated disclosures conflict in part or whole with, or differin scope from, the present disclosure, then to the extent of conflict,broader disclosure, or broader definition of terms, the presentdisclosure controls. If such incorporated disclosures conflict in partor whole with one another, then to the extent of conflict, thelater-dated disclosure controls.

The Abstract is provided as required as an aid to those searching forspecific subject matter within the patent literature. However, theAbstract is not intended to imply that any elements, features, orlimitations recited therein are necessarily encompassed by anyparticular claim. The scope of subject matter encompassed by each claimshall be determined by the recitation of only that claim.

What is claimed is:
 1. A semiconductor light-emitting device comprising:a semiconductor diode structure having front and back surfaces, thefront surface being characterized, with respect to an ambient medium, bya critical angle Θ_(C) at a nominal vacuum wavelength λ₀; one or morelight-emitting active layers within the semiconductor diode structurethat are arranged so as to emit active-layer output light at the nominalvacuum wavelength λ₀ to propagate within the semiconductor diodestructure; an anti-reflection coating on the front surface of thesemiconductor diode structure that is arranged so that the front surfaceexhibits reflectivity, for light incident on the front surface withinthe semiconductor diode structure at an incidence angle less than Θ_(C),at the nominal vacuum wavelength λ₀, and with the front surface againstthe ambient medium, that is less than corresponding Fresnel reflectivityof an interface between semiconductor diode structure material and theambient medium without the anti-reflection coating; and a redirectionlayer on the back surface of the semiconductor diode structure, theredirection layer including one or more of (i) an array ofnano-antennae, (ii) a partial photonic bandgap structure, (iii) aphotonic crystal, or (iv) an array of meta-atoms or meta-molecules, theredirection layer being structurally arranged so as to redirect at leasta portion of active-layer output light, incident within thesemiconductor diode structure on the redirection layer at an incidenceangle greater than Θ_(C), to propagate toward the front surface of thesemiconductor diode structure at an incidence angle with respect to thefront surface that is less than Θ_(C), the semiconductor light-emittingdevice exhibiting (i) transmission by the front surface, as deviceoutput light that propagates in the ambient medium, of first and secondportions of the active-layer output light propagating within thesemiconductor diode structure, the first portion without redirection bythe redirection layer and the second portion with redirection by theredirection layer, and (ii) a mean number of redirections per photonemitted by the active layer, by the redirection surface beforetransmission by the front surface, that is less than
 30. 2. The deviceof claim 1 wherein the semiconductor light-emitting device exhibits aphoton extraction efficiency that is greater than about 80.%.
 3. Thedevice of claim 1 wherein the redirection layer exhibits a redirectionefficiency, of the light incident within the semiconductor diodestructure on the redirection layer at an incidence angle greater thanΘ_(C) to propagate toward the front surface of the semiconductor diodestructure at an incidence angle with respect to the front surface thatis less than Θ_(C), that is greater than about 80.%.
 4. The device ofclaim 3 wherein the redirection layer exhibits optical loss per pass forlight incident thereon that is less than about 20.%.
 5. The device ofclaim 1 further comprising: a dielectric layer on a back surface of theredirection layer with the redirection layer between the dielectriclayer and the back surface of the semiconductor diode structure, thedielectric layer being, at the nominal vacuum wavelength λ₀,substantially transparent and characterized by a refractive index lowerthan that of the semiconductor diode structure; and a reflective layeron a back surface of the dielectric layer with the dielectric layerbetween the reflective layer and the back surface of the redirectionlayer.
 6. The device of claim 5 wherein the redirection layer exhibitsan efficiency of redirection, of the light incident within thesemiconductor diode structure on the redirection layer at an incidenceangle greater than Θ_(C) to propagate toward the front surface of thesemiconductor diode structure at an incidence angle with respect to thefront surface that is less than Θ_(C), that is greater than about 20.%.7. The device of claim 5 wherein the redirection layer exhibits opticalloss per pass for light incident thereon that is less than about 10.%.8. The device of claim 5 wherein the reflective layer exhibitsreflectivity, at the nominal vacuum wavelength λ₀, that is greater thanabout 90.%.
 9. The device of claim 5 wherein the reflective layerincludes a metallic coating or a dielectric coating.
 10. The device ofclaim 5 wherein the back surface of the dielectric layer includescorrugations, dimples, bumps, protrusions, or depressions.
 11. Thedevice of claim 1 wherein the redirection layer includes an array ofnano-antennae that include one or more antenna materials, are shaped,sized, and spaced relative to the nominal output vacuum wavelength λ₀,and arranged along the redirection layer, so as to reradiate, uponirradiation by active-layer output light at an incidence angle greaterthan Θ_(C), at least a portion of the active-layer output light toresult collectively in the redirection thereof to propagate toward thefront surface of the semiconductor diode structure at an incidence angleless than Θ_(C).
 12. The device of claim 1 wherein the redirection layerincludes a partial photonic bandgap structure arranged with one or morematerials, morphology, and spacing relative the nominal output vacuumwavelength λ₀, so as to redirect, upon irradiation by active-layeroutput light at an incidence angle greater than Θ_(C), at least aportion of the active-layer output light to propagate toward the frontsurface of the semiconductor diode structure at an incidence angle lessthan Θ_(C).
 13. The device of claim 1 wherein the redirection layerincludes a photonic crystal arranged with one or more materials, crystalmorphology, and crystal-lattice spacing relative the nominal outputvacuum wavelength λ₀, so as to redirect, upon irradiation byactive-layer output light at an incidence angle greater than Θ_(C), atleast a portion of the active-layer output light to propagate toward thefront surface of the semiconductor diode structure at an incidence angleless than Θ_(C).
 14. The device of claim 1 wherein the redirection layerincludes an array of meta-atoms or meta-molecules that include one ormore meta-materials, are shaped, sized, and spaced relative to thenominal output vacuum wavelength λ₀, and arranged along the redirectionlayer, so as to reradiate, upon irradiation by active-layer output lightat an incidence angle greater than Θ_(C), at least a portion of theactive-layer output light to result collectively in the redirectionthereof to propagate toward the front surface of the semiconductor diodestructure at an incidence angle less than Θ_(C).
 15. The device of claim1 wherein the redirection layer includes a diffuse backscattererarranged so as to backscatter, upon irradiation by active-layer outputlight at an incidence angle greater than the critical angle Θ_(C), atleast a portion of the incident active-layer output light to propagateat an incidence angle less than Θ_(C).
 16. The device of claim 1 whereinreflectivity of the front surface of the semiconductor diode structurewith the anti-reflection coating is less than about 10.%.
 17. The deviceof claim 1 wherein the semiconductor light-emitting device comprises asemiconductor light-emitting diode.
 18. A method for making asemiconductor light-emitting device, the method comprising: formingwithin a semiconductor diode structure one or more light-emitting activelayers that are arranged so as to emit active-layer output light at anominal vacuum wavelength λ₀ to propagate within the semiconductor diodestructure, the semiconductor diode structure being characterized withrespect to an ambient medium by a critical angle Θ_(C) at the nominalvacuum wavelength λ₀; forming on a front surface of the semiconductordiode structure an anti-reflection coating that is arranged so that thefront surface exhibits reflectivity, for light incident on the frontsurface within the semiconductor diode structure at an incidence angleless than Θ_(C), at the nominal vacuum wavelength λ₀, and with the frontsurface against the ambient medium, that is less than correspondingFresnel reflectivity of an interface between semiconductor diodestructure material and the ambient medium without the anti-reflectioncoating; and forming on the back surface of the semiconductor diodestructure a redirection layer, the redirection layer including one ormore of (i) an array of nano-antennae, (ii) a partial photonic bandgapstructure, (iii) a photonic crystal, or (iv) an array of meta-atoms ormeta-molecules, the redirection layer being structurally arranged so asto redirect at least a portion of active-layer output light, incidentwithin the semiconductor diode structure on the redirection layer at anincidence angle greater than Θ_(C), to propagate toward the frontsurface of the semiconductor diode structure at an incidence angle withrespect to the front surface that is less than Θ_(C), the semiconductorlight-emitting device exhibiting (i) transmission by the front surface,as device output light that propagates in the ambient medium, of firstand second portions of the active-layer output light propagating withinthe semiconductor diode structure, the first portion without redirectionby the redirection layer and the second portion with redirection by theredirection layer, and (ii) a mean number of redirections per photonemitted by the active layer, by the redirection surface beforetransmission by the front surface, that is less than
 30. 19. The methodof claim 18 further comprising: forming on a back surface of theredirection layer a dielectric layer with the redirection layer betweenthe dielectric layer and the back surface of the semiconductor diodestructure, the dielectric layer being, at the nominal vacuum wavelengthλ₀, substantially transparent and characterized by a refractive indexlower than that of the semiconductor diode structure; and forming on aback surface of the dielectric layer a reflective layer with thedielectric layer between the reflective layer and the back surface ofthe redirection layer.
 20. A method for operating a semiconductorlight-emitting device assembly, the method comprising supplying to thesemiconductor light-emitting device electrical power so that thesemiconductor light-emitting device emits device output light from afront surface of a semiconductor diode structure to propagate in anambient medium against the front surface: the semiconductor diodestructure having front and back surfaces, the front surface beingcharacterized, with respect to the ambient medium, by a critical angleΘ_(C) at a nominal vacuum wavelength λ₀; one or more light-emittingactive layers within the semiconductor diode structure being arranged soas to emit active-layer output light at the nominal vacuum wavelength λ₀to propagate within the semiconductor diode structure; ananti-reflection coating on the front surface of the semiconductor diodestructure being arranged so that the front surface exhibitsreflectivity, for light incident on the front surface within thesemiconductor diode structure at an incidence angle less than Θ_(C), atthe nominal vacuum wavelength λ₀, and with the front surface against theambient medium, that is less than corresponding Fresnel reflectivity ofan interface between semiconductor diode structure material and theambient medium without the anti-reflection coating; a redirection layeron the back surface of the semiconductor diode structure including oneor more of (i) an array of nano-antennae, (ii) a partial photonicbandgap structure, (iii) a photonic crystal, or (iv) an array ofmeta-atoms or meta-molecules, the redirection layer being structurallyarranged so as to redirect at least a portion of active-layer outputlight, incident within the semiconductor diode structure on theredirection layer at an incidence angle greater than Θ_(C), to propagatetoward the front surface of the semiconductor diode structure at anincidence angle with respect to the front surface that is less thanΘ_(C), the semiconductor light-emitting device exhibiting (i)transmission by the front surface, as device output light thatpropagates in the ambient medium, of first and second portions of theactive-layer output light propagating within the semiconductor diodestructure, the first portion without redirection by the redirectionlayer and the second portion with redirection by the redirection layer,and (ii) a mean number of redirections per photon emitted by the activelayer, by the redirection surface before transmission by the frontsurface, that is less than 30.